A photodiode is a type of photo-detector capable of converting light into current or voltage. Generally, the photodiode is a semiconductor device with a PIN or PN connection structure. When a photon of sufficient energy strikes the photodiode, it excites electrons, creating free electrons and positively charged electron holes. The holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced proportional to the amount of incident light on the photodiode.